The present invention relates to a friction stir welding apparatus and method and, more particularly, to a backing member and a method of using same in a friction stir welding process which facilitate total penetration of a butt weld as well as creation of a visual indication of the completeness of the weld.
Friction stir welding is a known technique for welding and has found particular utility for butt welding two workpieces together along a joint line therebetween defined by abutting edges of the workpieces. The friction stir welding process generally involves engaging the material of the two workpieces on either side of the joint by a cyclically moving stir pin or blade. Friction between the stir pin or blade and the workpieces causes frictional heating and plasticizing of the material adjacent the joint. The stir pin or blade is traversed along the line of the joint, plasticizing the material as it moves, and the plasticized material left behind the stir pin or blade coalesces and resolidifies to form a weld between the workpieces.
The friction stir welding process has demonstrated unique joining capabilities of aluminum alloys. In a conventional method, a rotating stir pin is plunged into the material of the workpieces on either side of the weld, with the distal end or tip of the stir pin extending close to but not all the way through to the bottom surfaces of the workpieces. The xe2x80x9crootxe2x80x9d zone between the tip of the stir pin and the bottom surfaces of the workpieces theoretically is welded by plasticized material being-forced downward through the joint to the bottom surfaces of the workpieces. However, when a butt weld is formed by the process, the xe2x80x9crootxe2x80x9d side of the weld is difficult to inspect. In particular, penetration of the weld completely through the joint to the root side has been difficult to determine with conventional inspection techniques such as radiographic and ultrasonic inspection. It is possible for the abutting edges of the workpieces in the root region to abut one another so tightly that even though they are not welded, conventional inspection fails to detect the faulty weld.
Attempts have been made to modify the friction stir welding technique to better assure a complete weld penetration through the root zone. For example, U.S. Pat. No. 5,611,479 issued to Rosen discloses a method for friction stir butt welding in which the faying edges of the two workpieces are chamfered away from each other adjacent the bottom surfaces to create an inverted V-shaped space adjacent the back-up bar which contacts the bottom surfaces of the workpieces. The plasticized material from the friction stir weld can enter the volume created by the chamfer at the bottom of the faying surfaces. Visual inspection for complete weld penetration is then possible since a completely penetrated weld will be evidenced by solidified material present in the chamfered space.
The disadvantage of Rosen""s method is that it necessitates additional work steps to create the chamfered edges of the workpieces. Furthermore, if the weld penetration is not complete, then there will be voids in the chamfered volume which will have to be filled in by additional welding and blending along the root side. Accordingly, the method is not as economical as conventional friction stir butt welding methods.
The present invention overcomes the drawbacks associated with known friction stir welding apparatus and techniques by providing an apparatus and a method for friction stir welding which create a visible bead on the root side when the weld is fully penetrated. Thus, presence of the bead indicates full penetration, and lack of the bead at any point along the welded joint line indicates probable failure of the weld to fully penetrate at that point.
To these ends, the invention provides a friction stir welding apparatus including a back-up bar having a working surface (i.e., the surface which contacts the workpieces) which generally conforms to the contours of the workpieces on either side of the joint, except that the working surface has a groove formed therein. The back-up bar is positioned under the workpieces with the working surface contacting the bottom surfaces of the workpieces and with the groove located directly beneath the joint and the longitudinal axis of the groove parallel to the line of the joint. Thus, when the stir pin is plunged into the material of the workpieces at the joint and pressure is applied between the stir pin and the workpieces, the pressure causes the material which is plasticized by the stir pin to be forced downward through the root zone of the joint and into the groove in the back-up bar where the material solidifies to form a bead which stands out from the bottom surfaces of the workpieces.
The width, depth, and shape of the groove can be varied to suit the particular application. For instance, the volume of the groove, which is primarily a function of its width and depth, generally is dictated by the thickness of the workpieces at the joint, and is made as small as possible to minimize reduction in joint thickness.